1. Field of the Invention
This invention relates to a process for the preparation of the high-quality needle coke from coal tar (hereinafter referred to as CT), coal tar pitch (hereinafter referred to as CP), which is separated from CT, or the heavy oil derived from coal, or the like, said needle coke being suitable for the preparation of the graphite electrode that is used in the ultra-high power operation (UHP-operation) required in the steel making in an electric furnaces and which is suitable for the preparation of graphite electrodes capable of withstanding quick melt conditions. It also relates to the process for the preparation of the high-quality needle coke or super needle coke also suitable for the preparation of graphite electrodes of the lengthwise graphitization system (LWG system) which recently has come into notice.
2. Description of the Prior Art
For realizing a graphite electrode that will withstand quick melt conditions for the UHP operation of the electric furnace and exhibit good performance in practice, it is required of the coke to have a low electrical resistivity and a low coefficient of thermal expansion (hereinafter referred to as CTE), while it is required of the graphitized product to have low modulus of elasticity and high strength. It is also required that, in view of the tendency towards the larger size of the electrodes, that the coke material be homogeneous in quality.
In order to meet such requirements, a notable improvement has been made in the quality of the so-called needle coke (hereinafter referred to as N-coke) derived from petroleum or coal sources. In view of the properties desired of the N-coke, it is also known under the name of easily graphitizable coke or high crystalline coke.
The carbon material generally prepared by coking the starting coking material typically at the coking temperature of 430.degree. to 470.degree. C., also known as raw N-coke or green N-coke, is composed of aggregates of graphitelike fine crystallites of hexagonal system with the mean size of the order of 1nm (10 .ANG.). The properties of the N-coke for the preparation of the high-quality graphite electrodes as mentioned hereinabove are known to depend on the orientation of and the binding force acting among these crystallites.
The formation of these crystallites is markedly affected in a known manner by the state of generation of the fine optically anisotropic mesophase spherules from which the bulk mesophase are formed by coalescence of small spherules and the growth thereof finally resulting in the coke precursors upon heating the starting coking material.
On the other hand, the mesophase spherules are affected by such factors as the composition of the starting coking material, impurities that obstruct the growth of the mesophase spherules, and the coking conditions, so that it is by no means easy to specify the N-coke structure.
However, the CTE is the independent property of the N-coke which is solely determined at the stage of the raw coke formation in the coking reaction, its history in terms of CTE being extended even after the graphitization stage and cannot improve any more.
For this reason, the current practice in the commercial circles is to mainly classify the grades of N-coke as a function of the CTE values.
Although the N-coke grades are not necessarily dependent solely upon the CTE values, as a general rule, those N-cokes having the CTE values, shown as an average value over the temperature range of 100.degree. to 400.degree. C., of the order of 1.00 to 1.15.times.10.sup.-6 /.degree.C., are indicated as premium-grade N-coke or PN-cokes, while those having the CTE value in the range of 1.15 to 1.25.times.10.sup.-6 /.degree.C. are indicated as the regular grade needle coke, regular N-coke or RN-coke.
Compared to the RN-coke, the PN-coke has a large crystal size, superior crystal orientation and a high real density, so that it may be said to be superior in graphitizability.
When the CT or the CP derived from CT is coked as such by direct coking, the resulting coke is notably inferior to the RN-coke and practically unusable for the preparation of the graphite electrode.
The essential conditions for the preparation of the high grade N-coke usable for the preparation of the graphite electrode for the purpose of UHP operation are meticulous sorting or selection and refining of the starting coking material.
For example, it is described in the Japanese Patent Laid Open Specification No.78201/1977 to separate or eliminate quinoline insolubles (QI) out of CP through selection of the ratio of the aromatic solvents mixed with CP and being coked the resulting material by the conventional delayed coking. It is described in the Japanese Laid-Open Patent Publication No. 28501/1977 to eliminate the QI components out of the hydrocarbon material containing said QI components and the condensed ring hydrocarbon compounds by using a solvent the 95 volume percent of which has the boiling point lower than 330.degree. C. and the BMCI value of which is in the range of 5 to 70, then to remove the solvent and being coked the resulting product by conventional delayed coking to the desired N-coke.
It should be noted that the methods described in these two publications are intended for QI removal and that, when the starting materials prepared from these known methods are used for coke manufacture, while it is indeed possible to obtain the PN grade coke in terms of CTE values, however, swelling or puffing phenomena was undesirably observed when using such coke for the preparation of the graphite electrode in accordance with the LWG system.
Such puffing phenomena is also seen to occur with the N-coke grade which is of substantially the same grade as that obtained from the petroleum sources. However, such puffing is mainly ascribable to the sulphur contained in the coke and, in general, may easily be controlled by the addition of iron oxides as anti-puffing agent. It should be noted that such puffing preventive measures are not effective in the case of the coking material derived from coal sources.
It is also known that the graphite electrode from the PN-coke manufactured from the material derived from coal sources is excellent in mechanical strength but slightly inferior in tenacity to the similar electrode derived from petroleum sources.
Although the reason for these defects is not known precisely, it is generally thought that gases desorbed from hetero atoms contained in the coke, such as N, 0 or S and the texture of the carbon material are playing some part in the course of the electrode graphitization.
The QI components present in the starting coking material accelerate the coking rate, but such material becomes affixed to the surface of the mesophase spherules in the course of the coking reaction and obstructs the mesophase growth, the coke texture thus obtained becoming the micro mosaic structure instead of bulk mesophase.
Further the bulk mesophase is not turned into the fibrous texture even upon heat treatment in the course of the subsequent coking reaction so that the resulting product is not the high grade N-coke suitable for the production of the graphite electrode.
It is therefore necessary that the QI contents in the starting material be removed from the starting coking material or be converted into components that are innocuous to the coking reaction.
Not withstanding the forgoings, the use of QI-free starting coking material does not necessarily give rise to a high quality N-coke, thus posing another problem.
This phenomenon is outstanding especially in case of using a starting coking material derived from coal, sources such as CT or CP.
For example, it is supposed that the QI components are removed by any suitable method from CT or CP to give QI-free CT(QI-F-CT) or QI-free CP(QI-F-CP) as starting coking material, which is then coked by conventional delayed coking at a pressure of about 0.3 MPa (3 kg/cm.sup.2 G) at a lower coking temperature of, for example, 440.degree. C. The coke thus obtained may have a CTE comparable to that of the PN-coke. However, when the same starting material is subjected to the coking reaction under the more higher coking temperature of, for example, 445.degree. C., 450.degree. C. or 460.degree. C., and other conditions being the same, the CTE of the resulting coke is of the same order of magnitude as or even inferior to that of the RN-coke. Thus, with rising in the coking temperature, the CTE value is increased rapidly while the coke properties are notably lowered.
In this connection, it may be surmised that certain ingredients contained in the QIF-CT or QIF-OP are not harmful to the formation of good bulk mesophase with good fibrous texture when coked under a comparatively lower coking temperature, but which obstruct generation of the bulk mesophase with the fibrous texture as the coking temperature is increased because of the coking rate of such ingredients then become large.
Although it is difficult to discern or specify such components responsible for such behaviors, this unidentified substance is referred to herein as DRRC (dormant rapid reaction component excited by temperature).
For preparing the high-quality N-coke from the DRRC containing starting material, it is necessary to convert DRRC into components innocuous to the coking reaction or to remove DRRC out of the system in any way to prevent DRRC from taking part in the coking reaction. Delayed coking at an elevated coking temperature becomes possible only subject to such a treatment as stated above.
It is thought that some DRRC may be inherently an intrinsic component of the QIF-CT or QIF-CP, while the other DRRC may be subsequently formed during the course of preliminary heat treatment or in the course of coking reaction.
It will be noted that about 10 and 20 weight percent of n-heptane insolubles (hereinafter referred to as nC7-I) are contained in QIF-CT and QIF-CP, respectively. This nC7-I is a mixture with complex chemical structures of polycondensed aromatic compounds with polyfunctional groups inheriting the chemical structure of coal.
The nC7-I can be separated into toluene soluble components (hereinafter referred to as TS), and toluene insoluble components (hereinafter referred to as TI), amounting to ca. 6.5 to 10 percent and 3.5 to 10 percent, respectively.
TI components of asphaltenes are soluble to quinoline, also known as pre-asphaltenes, are a high molecular weight material containing about 4 percent of hetero atoms, mainly oxygen atoms.
The TS components also contain about 4 percent of hetero atoms. The nC7-I derived from petroleum sources differs in the respect that it essentially consists only of TS components and it is mainly composed of C and H.
Unexceptionally, these undergo gradual changes in their chemical structure by hydrogenation or thermal cracking. In view of the fact that starting coking material derived from thermally cracked oil which, substantially free of nC7-I or TI components thereof, does not show the DRRC-induced phenomena during the coking reaction, and in addition the heteroatoms present in the starting coking material generally obstruct the formation of the highquality coke, it is thought that contained in QIF-CT or QIF-CP, if involved thereof components that exhibit the function heretofore described as DRRC.
Since the coking reaction proceeds associated with numerous components subjected to a strong intermolecular reaction, it has not been feasible to make a scrutiny of these by looking at individual components.
As we investigate into the conditions leading to formation of SN-coke (super needle coke) through modifying or excluding materials which induce DRRC, the percentage of the conversion or reduction of the nC7-I and TI based on those contained original QIF-CP under the relatively moderate hydrogenation conditions with the denitrogenation (de-N) percentage in the hydrocracked oil based on the nitrogen content of original QIF-CP equal to 15 percent were 21.4 and 38.6 percent respectively. These values amounted to 62.5 and 74.5 percent under the severe conditions when the de-N percentage equals to 80 percent.
It is obvious from above that, while the amounts of nC7-I and TI could be reduced by hydrogenation, it is still difficult to completely convert or reduce them into other components solely by hydrogenation.
The distributions of the nC7-I and TI components in the hydrogenated oil derived from QIF-CP is such that trace amounts of nC7-I are observed in the 350.degree. to 521.degree. C. cut or fraction for the de-N percent of 15 percent and the nC7-I and TI remained are found to be distributed in the fraction of 521.degree. C. to the heavy-end when the de-N percent higher is than 15 percent.
On the other hand, the amount of heavy ends in the same hydrogenated oil with the boiling range above 521.degree. C. is expectedly decreased with increase in the de-N percentage. That is, for the de-N percent in the range of 15 to 80 percent, the conversion or reduction ratio to the same heavy ends of the same boiling range of QIF-CP amounted to 44 to 60 percent.
The contents of nC7-I and TI in the same heavy ends of the hydrogenated oil amounted to 44 to 30 percent and 16 to 10 percent, respectively, meaning that much nC7-I and TI are yet contained in the heavy ends.
There is described in the Japanese Patent Publication No. 11442/1974 the method of modifying the coal tar pitch by hydrogenation to a pitch material having a chemical structure likely to produce easily graphitizable needle coke. However, the SN coke cannot be obtained even if the material produced in this manner is used as such as the starting coking material.
In the Japanese Patent Publication No. 41129/1976, there is described the method for the preparation of the pitch coke from the tar pitch derived from petroleum sources and that derived from coal sources.
According to this method, the starting tar pitch is alkylated and thereafter modified in the presence of the hydrogenation catalyst.
However, by these methods, the QI components are still contained in the starting coking material so that it is not possible to obtain the starting coking material for SN-cokes schemed to provide by the present invention.
The thermal cracking subsequent to hydrogenation results in a further increase in the percentage of conversion due to cracking or reduction of the heavy ends in the hydrogenated oil. The overall cracking or reduction percentage based on the heavy end portion of the QIF-CP as a result of the hydro- and thermal-cracking amounts to 67.5 to 72.5 percent for the de-N percent of 15 to 80 percent, which means a further increase of 23.5 to 15 percent points over the value obtained by hydrogenation.
On the other hand, the overall conversion or reduction percent of the nC7-I amounts to 20.3 to 60.5 percent whereas that of TI amounts to 26.9 to 75.3 percent. Thus the value for nC7-I is apparently nearly equal to that obtained by hydrogenation, while that for the TI component is decreased about 10 percent below the value obtained by hydrogenation for the de-N percent of 15 percent, but it is substantially not changed for the de-N percent of 80 percent.
The conversion or reduction percentage of the heavy ends with the boiling point above 521.degree. C., obtained upon direct thermal cracking of QIF-CT or QIF-CP but without hydrogenation, is about 50 percent at most, whereas the conversion or reduction percentage of the former is only 7 percent and that of the latter is increased to more than twice. Even if the thermal cracked oil obtained in this manner is processed as described above, it has been completely impossible to obtain as middle cut the starting coking material free of nC7-I and TI or DRRC.
Therefore, in order to process QIF-CT or QIF-CP to produce the starting coking material for SN-cokes, both the hydrogenation and the thermal cracking contiguous thereto are indispensable, or inseparable from each other. Although DRRC can be separated by the hydrogenating step alone, subject to a suitable selection of the de-N percentage, the coke yield of the middle cut as the starting coking material obtained by subjecting the hydrogenated oil in situ to flashing is extremely low, as is the practical value of such starting coking material. On the other hand, thermal cracking alone is not subservient to the object of the present invention because it fails to lead to complete DRRC separation.